Trough solar systems use parabolic, single-axis tracking mirrors to concentrate sunlight 30-60 times onto an evacuated receiver tube, which runs the length of the trough at the focal line of the reflector, thus heating the heat-transfer fluid (i.e., synthetic oil) flowing through the receiver. Electricity is generated by passing the heated oil through a heat exchanger and using the steam generated to drive a commercial turbine generator. Collectors axe typically aligned on a north-south axis, and the trough tracks the sun from east to west during the day to maximize the sun's energy input to the receiver tube. Parabolic-trough solar technology has been demonstrated by nine utility-scale plants installed between 1984 and 1991 in California's Mojave Desert. These plants, referred to as Solar Electric Generating Systems (SEGS), represent 354 megawatts (MW) of installed electric-generating capacity that operate daily, providing power to the electricity grid. The SEGS experience has proven the parabolic-trough technology to be a robust and reliable power technology in an industrial-utility operating environment. Its key advantages are proven performance, manufacturing simplicity, use of standard equipment and materials, improvement in cost effectiveness via incremental steps, and low technical or financial risk to the investor.
Experience from the SEGS plants has shown that the reliability and lifetime of the parabolic-trough collector receiver tube or heat-collection element (HCE) is the most significant operating and maintenance issue for existing and future parabolic-trough plants. HCE designs presently use an evacuated receiver fabricated from stainless-steel tubing with a cermet coating (cermets are highly absorbing metal-dielectric composites containing fine metal particles in a dielectric or ceramic matrix), a Pyrex® glass envelope externally and internally coated with an antireflective (AR) coating, and conventional glass-to-metal seals. For perspective, each receiver tube is usually about 4 m (13.1 ft) long and 70 mm (2.25 in) in diameter.
The overall solar-to-electric efficiencies of parabolic-trough solar power plants can be improved and the cost of solar electricity could be reduced by improving the properties of the selective coating on the receiver and increasing the solar-field operating temperature to above 400° C. New, more-efficient selective coatings will be needed that have both high solar absorptance and low thermal emittance at such elevated temperatures. Although the present coatings are designed to be used in evacuated environments, the coatings need to be stable in air at elevated temperatures in case the vacuum is breached. Current coatings do not have the stability and performance desired for moving to higher operating temperatures. For efficient photothermal conversion, solar absorber surfaces must have low reflectance (ρ=0) at wavelengths λ≦2 μm and a high reflectance (ρ≈1) at λ≧2 μm. The reflectance cutoff may be higher or lower, depending on the operating temperature. For parabolic-trough applications, an improved spectrally selective surface should be thermally stable above 450° C., ideally in air, and have a solar absorptance (α) greater than about 0.96 plus a thermal emittance (ε) below about 0.07 at 400° C. Achieving the improved properties is very important if the parabolic trough systems are going to be operable at higher temperatures.
At this point, none of the existing prior art coatings used commercially have proven to be stable in air at 400° C. Designing and fabricating a solar-selective coating that is stable in air at temperatures greater than 450° C. requires high thermal and structural stabilities for both the combined and individual layers, excellent adhesion between the substrate and adjacent layers, suitable texture to drive the nucleation and subsequent growth of layers with desired morphology, enhanced resistance to thermal and mechanical stresses, and acceptable thermal and electrical conductivities. Other desirable properties of good continuity and conformability over the tube, as well as compatibility with fabrication techniques. The material should have a low diffusion coefficient at high temperature and be stable with respect to chemical interactions with any oxidation products, including any secondary phases present, over long periods of time at elevated temperatures. Selecting materials with elevated melting points and large negative free energies of formation may meet these objectives. Stable nanocrystalline or amorphous materials are the most desirable (and practical) for diffusion-barrier applications, especially in light of material and process limitations. However, there will likely be trade-offs in the microstructure between a highly oxidation-resistant coating (i.e., amorphous or nanocrystalline) and a solar-selective coating with both high absorption (i.e., columnar or porous microstructure) and low emittance (i.e., smooth or highly dense). High thermal stability is manifested by high melting points, single-compound formation, and lack of phase transformations at elevated temperature.
Solar-selective “absorber” coatings can be categorized into six distinct types (as shown in FIGS. 2A through 2F below): 2A) intrinsic, 2D) semiconductor-metal tandems, 2C) multilayer absorbers, 2D) multi-dielectric composite coatings, 2E) textured surfaces, and 2F) selectively solar-transmitting coating on a black body-like absorber. Reviews of the literature revealed selected properties of some materials in these categories. Intrinsic absorbers use a material having intrinsic properties that result in the desired spectral selectivity and include: metallic W, MoO3-doped Mo, Si doped with B, CaF2, HfC, ZrB2, SnO2, In2O3, Eu2O3, ReO3, V2O5, and LaB6. Semiconductor-metal tandems absorb short-wavelength radiation because of the semiconductor band gap and have low thermal emittance as a result of the metal layer. Semiconductors of interest include Si (1.1 eV), Ge (0.7 eV), and PbS (0.4 eV). Multilayer absorbers use multiple reflections between layers to absorb light and can be tailored to be efficient selective absorbers. Metal-dielectric composites—called cermets—contain fine metal particles in a dielectric or ceramic host material. Textured surfaces can produce high solar absorptance by multiple reflections among needle-like, dendritic, or porous microstructures. Additionally, selectively solar-transmitting coatings on a black body-like absorber are also used, but are typically used in low-temperature applications.
Multilayer absorbers or multilayer interference stacks can be designed so that they become efficient selective absorbers (see FIG. 3). The selective effect arises because the multiple reflectance passes through the bottom dielectric layer (E) and is independent of the selectivity of the dielectric. A thin semitransparent reflective layer (D), typically a metal, separates two quarter-wave dielectric layers (C) and (E). The bottom-reflecting layer (D) has high reflectance in the infrared (IR) region and is slightly less reflective in the visible region. The top dielectric layer (C) reduces the visible reflectance. The thickness of this dielectric determines the shape and position of the reflectance curve. An additional semitransparent (i.e., thin) metal layer (B) further reduces the reflectance in the visible region, and an additional dielectric layer (A) increases the absorption in the visible region and broadens the region of high absorption. The basic physics of the multilayer absorber is well understood by those skilled in the art, and computer modeling can easily compute the optical properties given by an optimum multilayer design of candidate materials. Multilayer interference, stacks provide high solar absorption and low thermal emittance, and can be stable at elevated temperatures (up to about 400° C.), depending upon the materials used. Several multilayer absorbers using different metals (e.g., Mo, Ag, Cu, Ni) and dielectric layers (e.g., Al2O3, SiO2, CeO2, ZnS) are known for high-temperature applications.
Metal-dielectric composite coatings or absorber-reflector tandems have a highly absorbing coating in the solar region (i.e., black) that is transparent in the IR, deposited onto a highly IR-reflective metal substrate (FIGS. 4A through 4C below). The highly absorbing metal-dielectric composite, or cermet, contains fine metal particles in a dielectric or ceramic matrix, or a porous oxide impregnated with metal. These films are transparent in the thermal IR region, while they are strongly absorbing in the solar region because of interband transitions in the metal and the small particle resonance. When deposited on a highly reflective mirror, the tandem forms a selective surface with high solar absorptance and low thermal emittance. The high absorptance may be intrinsic, geometrically enhanced, or both. The absorbing cermet layer comprising inherently high-temperature materials can have either a uniform or graded metal content. The metal-dielectric concept offers a high degree of flexibility, and the solar selectivity can be optimized by proper choice of constituents, coating thickness, particle concentration, size, shape, and orientation. The solar absorptance can be boosted by suitable choices of substrates and AR layers, which can also provide protection (for example, from thermal oxidative degradation). A variety of techniques, such as electroplating, anodization, inorganic pigmentation of anodized aluminum, chemical vapor deposition (CVD), and co-deposition of metal and insulator materials by physical vapor deposition (PVD), can produce the composite coatings. A subclass of this category is a powdered semiconductor-reflector combination, where the solar-selective properties of semiconductor, inorganic metal oxides, organic black pigments, and metal-dust-pigmented selective paints can be considered. FIG. 4A shows a metal substrate with attached columns or rods of metal which are encased in a dielectric matrix.
In a graded cermet (FIG. 4B), the reflectance from the cermet is reduced by gradually increasing the metal volume fraction, hence the refractive index of the material, from top to bottom, as a function of depth from the surface to the base of the film. PVD or CVD techniques can be used to form most graded cermets. By controlling the PVD deposition parameters, the microstructure of the oxides can be deposited with a porous to columnar microstructure, and by codeposition the inclusions or pores can be tilled with metal by evaporation of sputtering.
A double-cermet film structure has been developed through fundamental analysis and computer modeling that has higher photothermal conversion efficiency than surfaces using a homogeneous cermet layer or a graded film structure. Solar radiation is effectively absorbed internally and by phase interference in double-cermet solar coatings. Further, it is easier to deposit the double-cermet selective coating than graded-cermet layer selective surfaces. The typical double-cermet layer film structure shown in FIG. 5 from surface to substrate consists of the following: an AR layer that enhances solar absorption; an absorbing layer composed of two homogenous cermet layers, a low-metal-volume fraction (LMVF) cermet layer on a high-metal-volume fraction (HMVF) cermet layer; and a metallic infrared reflector layer to reduce substrate emittance. Similar to the double-cermet structure, 4-layer cermets, where the cermet compositional gradient metal volume fractions (VF) vary from about 0.5 to 0.8 have been made, modeled and prepared with good spectral selectivity.
Surface texturing is a common technique used to obtain spectral selectivity by the optical trapping of solar energy. Properly textured surfaces appear rough and absorb solar energy while appearing highly reflective and mirror-like to thermal energy. The emittance values can be adjusted (higher or lower) by modifying the microstructure (microcrystallites) of the coatings with ion-beam treatments. Even single-material surfaces can exhibit selective properties if they have the proper roughness, because the selective properties depend on the ratios of mean height deviations and the autocorrelation distance to the wavelength. Needle-like, dendrite, or porous microstructures on the same scale as the wavelength of the incident radiation exhibit both wavelength and directional selectivity. This geometrical selectivity is not very sensitive, however, to the severe environmental effects (i.e., oxidation, thermal shocks) that can have catastrophic influence on the lifetime of conventional multilayer selective coatings.
In the art of thin-film growth, it is well known that columnar microstructure will grow depending on the material itself and the deposition conditions-substrate temperature, deposition rate, vacuum pressure, and angle of incidence during deposition. It has been determined that evaporation at oblique angles drastically changed the film properties. FIG. 6 illustrates this phenomenon, showing a substrate upon which a film is being deposited by vapor deposition, where angle α is the flux arrival angle measured from the substrate normal and angle β is the columnar microstructure inclination angle, also measured from the substrate normal. It has been found that the direction and magnitude of the magnetic properties wave are dependent upon the angle of incidence, specifically when the substrate was tilted through a 45° angle between the evaporation flux and a substrate of Permalloy™ (alloy containing approximately 79 percent nickel and 21 percent iron) on large (3 in2 substrates and the deposition geometry, as shown in FIG. 6. Recently, glancing angle deposition (GLAD), illustrated in FIG. 7, employing oblique angle vapor flux deposition and substrate motion has been used to engineer thin film microstructures on a nanometer scale in three dimensions. However, rotating large substrates such as the large receiver tubes used in solar collectors around their lengths as in this technique is not feasible.
The columnar microstructure can be tailored with a wide range of control dependent on the material and deposition conditions by controlling the substrate relative to the impinging flux. The surface of the microstructure must be protected from damage caused by surface contact or abrasion. Selection of a material having a high intrinsic absorption coefficient can further optimize the absorptance. Dense arrays of tungsten whiskers have good selectivity, as do textured stainless-steel. Textured copper, aluminum, nickel, and chrome were reported by other sources not to have the necessary thermal stability in air. Metals that are reportedly stable in air at high temperatures (>600° C.) include molybdenum, rhodium, platinum, tungsten, hafnium carbide, and gold. The oxides of nickel and cobalt have been observed to be stable in air above 800° C. Amorphous silicon, germanium, and GaAs have been textured with hydrogen peroxide or by reactive ion or sputtering etching to produce highly absorbing selective surfaces.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.